Apparatus and method for regulating attenuation and phase on transmission lines



H. NY'QUisT l. v

Feb, 1, 1927. N 1,615,911

- -APPARATUS AND METHOD FOR REGULATING ATTENUATION AND PHASE ONTRANSMISSION LINES` l/ff 1 j@ f INVENTOR A TTORNE 1/ 1,615,911 Feb. 1,1927. H NYQUIST l APPARATUS AND METHOD FOR REGULATING ATTENUATION ANDPHASE ON TRANSMISSION LINES v Filed Feb. 29, 1924 v 4 Sheets-Sheet 2 [NVEN TOR I By www ATTORNEY Feb. 1, 1927. 1,6159

' H. NYQUIST APPARATUS vANDl METHOD FOR REGULATING ATTENUATION AND PHASEON TRANSMISSION LINES Filed Feb. 29, 1924 '4 Sheets-Sheet 3 0 500 m004500 v w00 A TTORNE Y Feb. 1 1927-.

A-PPARATUS AND METHOD FOR REGULATING ATTENUATION -AND PHASE 1,615,91l H.NYQUIST ON TRANSMISSION LINES Filed Feb. 29, 1924 4 sheets-sheet 4 E 7{M/0m Frei/Jamey y 200 000 000` '800 /000 1200 .i400

A N VEN TOR m5/WM TTORNE Y Pawnee Feb. 1, 1927.

vUNrrED STATES PATENT'OFFICE.

HARRY N'YQUIs'nor JACKSON HEIGHTS, NEW Yomr, AssIGNon 'ro AMERICAN TELE-rnoNE AND TELEeEArH COMPANY, A coRroEATIoN or NEW Yonx.

A PiPARATUS .ND METHOD FOR REGULATING ATTENUATION AND PHASE O N TRANS-MISSION LINES.

Application led February 29, 1924. Serial No. 696,049.

. It is an object of my? invention to provide a new and im'rovetransmission `system adapted to maintain the current intensityv and thephase relations substantially constant at the receivingstation over aconsiderable frequency range. .Another object of my invention is toprovide for keeping' constant phase relations at the receiving stationin a transmission system notwithstanding varia- -tions of resistance dueto temperature changes or other causes onthe line. These and variousAother `objects of my invention will'become apparent on consideration ofa limited number of examples of the invention which I have chosen forillustration and which I now proceed to disclose in the fol# lowingspecification taken with the accompanyng drawings. It willbeI understoodthat the following disclosure relates to these examples of the inventionand that the scope, of the invention will be defined in the api pendedclaims.

Referrlng to the drawings, Figure 1 is a diagram of a loaded line towhich ref erence will he made in discussing the principles involved inmy invention; Fig. 2 is a diagram of a'transmission system emh'odying myinvention; Fig. 3 is a diagram of a potentiometer employed in vthesystem of Fig. 2; Figs. 4 and 5 are diagrams of-imned-- 4ancecombinations that may be .considered for the steps of potentiometer;Flg. 6 shows impedance asa" function of frequency for the combinationsof Figs. 4 and 5; Fig. 7 isV Ianother impedance combination alterna-tiveto Figs. 4 and 5: Fig. 8 shows impedanceas a function of frequency forthe main part of the potentiometer and Figs. 9, 10 and 11 show special*alternative. designs -fr this -main part. p On an alternatmg currenttransmission l line, as the resistance changes due to change oftemperature, it is Well known that the attenuation also changes, andmoreover, 1f

there are 'various frequency components in the current, their phaserelation at the re. ceiving end will be different for differentresistances. It lis an object of my invention to providev means foradjustment, so that whatever the changes in resistance on the linewithinnormal limits, the received currents will. always come in with a certainintensity and with unchanging phaserelations among their components. Itis important to maintain local sources of current at `the receiving endthat shall be accurately in step with the respect-ive components of thereceived carrier current, and to this end it is important that thosecomponents shall'not vary in relative phase with changes in resistanceon the line. r

Fig. 2 shows a transmission system with a plurality of repeaters at astation. A 22- ty'pe repeater is shownwith the balancing networks N. Inall of these repeaters the amplifying elements are designated A. Thepair of conductors 11 is one pair of a fourwire line on which thetransmission is from west to east, and the corresponding pair fortransmission from eastto west is designated 12. On each Vof these pairsthere may be a plurality of carrier currents of respective diii'erentfrequencies and on each of these frequencies there may be severalmessage channels. with phase discrimination and magnitudediscrimination.

The foregoing example indicates the importance of transmitt-ing'withconstancy of current intensity and constancy in the phase 'relations atall frequencies at the receiving suitable distance each way, as at 15and 16,

the conductors 13 are interrupted and their ends joined together so asto forma complete circuit on the battery 14 through the winding 17 offaVrelay at theA same `station with the battery 14 as shown in thedrawing.

The points 15 and 16 may be at any suitable distance, which may beseveral times the interval between two consecutive repeater stations onthe line.

Whatever the' change in the resistance of the conductors 11 and 12,there will be a corresponding change in the resistance ofthe conductors13, in the same cable with 11 and 12. The change of resistance in theconductors 13 will produce a change of the current in the circuit of thebattery 14.' By means-of thebiasing winding 18, anormal neutral positionwill be established for the relay armature 19. 4

On the input side of each repeater amplilier at the station is apotentiometer 20, and the movable contacts vof all the potentiometers 20are mechanically connected to be adjusted by displacement of thecrosshead 21. The Wheel 22 is rotated continuously by the motor 23 andis adapted to be displaced by the armature of the relay 19, so that .oneWay it engages to move the crosshead 21 one Way, and for displacement ofthe armature 19 the other Way, the wheel 2 2 engages to move thecrosshead 21 the other way. Thus it will be seenthat any changes in theresistance of the pilot condnctorsl, such. as due to temperaturechanges, lead to a' change in the current iowing through the relayhaving the Winding 17, and this relay adjusts the potentiometers so asto bring the current back to the normal value in the conductors 13, andat the same time the relay adjusts all the input potentiometers 20 forthe 'repeater amplifiers atthe station.

These potentiometers 20- varedesigned in such a Way that at alladjustments they keep the attenuation constant over the length of linefrom 1'5 to 16, and they keep the phase displacements constant for allessential frequencies in the transmission between the points 15 and 16.

C, theinductance perloadingf section is L and the resistance per loadingsection is a. Assume that in one of the loading sections .1 smalladditional resistance Ar is inserted and let us investigate the ellecton the received current. Assume that the impedance looking one Way is KEand lookingr the other Way is KW.` The total impedance before insertingthe resistance r is KTH-KW and the total impedance after inserting theresist-` ance is KE-l-KW-l-Ar.

Hence the resultant eiect on the .received current will be given by thecomplex ratio R, lWhere KJFSL (1) KE-i-KW-i-Ar '1 0 a closeapproximation, since Ar is relatively small, it follows that Thevalve ofKE+KW varies along the load-' ing section, being a minimum at theloading coil and a maxlmum at the midpoint of the section. Assume thatthe increment A1',

instead of being lumped as in Fig. 1, is distributed uniformly ovei theloading section; then instead of equation (l2), We shall have A y dr 10gRr o am; 3)

Ar in each oi. n loading sections, the corresponding equation 1sAwhereinl Am is the total increment of vresistance in all the sections.AThe real part ot equation (5) gives the total attenuation in napiersand the imaginary part gives the total phase change in radians. Y

To le'et a practical idea of the actual changes involved in such a caseas that under consideration, consider a medium heavy loaded 19-gaugesidecircnit with 1,000 loading sections and assume that the resistanceincrement is 1 ohm per loadingl section, that is, assume that A1121.Reasonable values under the/foregoing assumptions Will be thatL/Czl5402: I11:100 (approximately); and L:0.1T5. Substituting thesevalues, equation. (5) becomes 1000 10g R: I 10o for the respective phaseshitt 1O 39, 0o 51,

and 0o 34:. 1

From these valuesit is apparent that in the case of medium heavy loadedcircuits the phase displacement is probably small enough practicable.

so -that it can be taken care of by manual But for an extra. lightloaded adjustments.

19-gauge side circuit the .case is different; liereL/Cz7802, rzlOO, andL:0.045. Proceeding inthe same way as for the case of the medium heavyloaded 19-'gauge cuit,

are considerably greater than those obtained for medium heavy loading,and so great as to make manual adpistments quite im Referring to thepotentiometers 20 of Fig. 2, it will be seen that these face amplifiersare of the audion type, and have very high input impedances; hence thecurrent drawn from a potentiometer to its amplifier is negligible andthe voltage, impressed on Ythe amplifier is proportional to theimpedance of the part'of the potentiometer'within the amplifier inputterminals. Let this impedance at any adjustment be denoted by Z. Theeffect on the input voltage of the amplifier due to` a change AZ may begiven by the complex ratio` f Z-l-AZ and since'AZ is small compared toZ, it is approximately true that side cirwe find that for frequencies of500,

'- 1 00() and 1,500,the phase shifts are, respectively, 10o 3, 6 2', and4 11. These values l A11 increment of resistance Am` on the'line woulddecrease the receivedcurrent in a certain rato R as expressed inequation (5)'. It is desired to increase the amplifier input voltage inthe reciprocal ratio sos to conipensate andlzcep the `current constant.The condition foi` this is that Rzl/R oi' log RI-log R and accordinglyfrom equations` (5) and (8) it follows that Z L' r Mehta@ Fui-ther, tofacilitate design and within the freedom of choice aorded by theforegoing difference equation, we impose the condition that the topste-ps of the potentiometer 20 shall approach pure resistances,in otherwords, that the last few increments AZ shall approach a real value. Thisgives for the 'value of the impedance of the potentiometer at its toptap where M is an arbitrary real constant. Putting equation (9) in theform of a di'erential equation and solving, and making use oftheboundary condition afforded by equation (10), the result is obtainedthat The assumption was Amade that the top few steps of thepotentiometer are real. But having passed to a differential equation,these become infinitesimal, and the top finite step in Fig., 3 is seento be other than a pure resistance, though its reactancecompo'nent issmall as will appear presently.

` If Z is chosen according to equation (11) for all the steps.l theregulating network will peratui'e and at the temperature underconsideration. By putting p equal to a large, value in `equation it willlbe apparent that the expression l will equal the whole difference inattenuation for a high frequency; accordingly, this expression will bedenoied by the svmbol aA Also, for conve1ic1ico,tlie expresSionYr/L willbe denoted by the symbol Z). Equation (5) then takes the form 7While 7)may no determined from the measurements of 1' and L by taking theirratio-.- it may be preferable to determine b other;4k wise as oiitli'nedbelow and thereby' compensate for approximations involyed in derivingequation (12). To do this,-nieasure the at-L tenuation and phasechangefor two temperatures and for a 4number of frequencies, including somerather high values of fre.-

fluency, where I1 is small in comparison with p. Fromtheseiiieasurements compute the values for the complex ratio R. Thevalue ot' the logarithm of R is obviously equal to -a for large valuesof p. Substituting the values for R and a in equation (12) and solvingfor b, the result is obtained.

If these values for 7i are computed and plotted as a function of' p,they will give a substantially horizontal line.

Owing to the manner in which I) enters equation (12), it will beapparent that an ei'ror inthe choice of b has the san-ie effect Thisgives the' total impedance forV the potentiometer on any tap. At the toptap a should be zero. Assuming that the potentiometer steps ai'e to bein so-called standard miles this-impedance correctly to three terms aretwo, and each of them consists of one condenser and two resistances. Thefirst ot these is made up of a resistance A in series with a parallelcombination' comprising another resistfance Band a capacity C', as shownin Fig. 4. The other net work will consist of a resistance E andcapacity F in series` this combination being connected in parallel withanother resistance "D, as

shown in Fig. o. The impedance expressions for these net works can eachhe written out ill) in a power series corresponding ro that of ,equation(16), and the first three terms ot' each series caiithen be respectivelyequated to the corresponding,terms in equation (16) the resultingequations may be solved tov obtain the values for A, B and C or D, E andl", as -the case may he. Carrying this work through, the followingresults are obtained:

The simplest networks that will reproducev at high frequencies, and thatwe are dealing j withv the extra light loaded 19-gauge side circuitheretofore specified, a should have the value 0.109 at the second tap,0.218 at the third tap, 0.327 at the fourth tap, etc.

An obvious i'ocedui'e to compute the individual inipeeance steps wouldbe to compute .the values of' Z from equation (14) for any twovadjacentvsteps and take the difference of pthe two results; It 4will make thecomputation simpler and lead to substantiallyfthe saine result to obtainthe first derivative of equationi(14) with respect to a at the midpointof the step, and multiply that derivative by the increment of a, whichis 0.109. This gives for the increment corresponding to any one step AZ0.109M

= Vaca/ima (15) llore a is to be given the values 0.100 x 0.5,v

given in equation (15), and the result is obtained as a'power series ini (a-wFe-m) (15) Substituting the values for a and the value for inthese expressions, it becomes possible to compute the correspondingnetwork elements as they are designated -in Figs. 4 and 5. 10 get anidea as to how closely theoresulting networks obtained from equations(17) to (22) would simulate the Videal networks specified by equation(16), computations have been carried out Jfor two values of a, namely0.8 and 1.6, using the value 52100/0045, which is approximately' correctfor extra light loaded 19-gauge side circuits. The results are shown inFig. 6. The curves marked Y v required by equation (1(5). marked Y, areobtained by using the three element networks of Fig. l or Fig. 5 asdefined by equations (17) to (22 The curves marked Y2 are obtained bymaking use of a two element network as shown in Fig. 7, whose elementsare determined by equations (17) and (10). lt will be apparent 'fromFig. 6 that for the important part ot' the frequency range,'the threeelement network of Fig. 21 or Fig. 5 gives very close approximation tothe ideal of equation (16) The curves are the ideal curves with resultsas follows:

A. B. C'. SteP- ohms. ohms. M. F.

2130 78.8 7.75 1010 220 2.08 1710 342 1. 93 1540 Y 44s 1.54 1300 53s 1.33 1240 017 1.22 1110 085 1.15 094 745 1.11 890 707v 1.00 700 S42' 1. 09710 884 1.10 042 922 1.12 570 958 1.15 517 995 1.18 403 1030 1.23 4151070 1.28 372 1110 1.34 v334 1170 1.41 300 1230 1.40` 209 1310 1.58

From the expression 3 0, which occurs in'some of the equations (17) `to(22), it will lbe seen that these equations break down `Where' a='3 andbeyond this point, for the reason that the expressions will then call ffor negative values for either the resistance *or the capacity. However,this is not a serious ob]ection because the value (1:3 cori Then theexpression for the impedance of the circuit of Fig. 10 is expanded.,giving thev i' following:

" Zno alfa/iii the first four coefficients of d ip ' vEquatingVVequationsv (24) and (25), and solving, the

followin values are obtained for the elements o the network of Fig. 10.

c :0.00221 ld :0.000236'; Y e'=0.3651.

y These numerical vvalues were next substi- Athe curves are in tuted inthe expression for the impedance lof-thenetwork of Fig. 10 and theresulting-j alu'es of impedance were plotted on Fig.

TheA value for the arbitrary constant M whchfhas been vused above is20640. The -fo re mg lvalues of a,d and e are multiplieiby 20640,'andthe value of c is divided number of steps beyond this main 'part is 20.

Ve turn to equation (14) and substitute .for (L the value10.109 X20,jor2.18. VEquation (14) then` becomes A Z2O Y 1,l 1(2.-2V.isv(1-Lbf1p)1`/7The same valu-e of b heretofore mentioned, namely .100/0.045 has beenused to evaluate equation (23) and the result is plotted in full linesin Fig. 8. From these full line curves it is inferred. that networks ofthe type shown in one of Figs. 9, and 11 could vbe made to simulate thisimpedance closely; 'lhey have all been investigated carefully and Fig.10 has been adoptedas mostsatisfactory. The procedure for Fig. 10 willbe given and may be taken as an example for all three Figs. 9, 10 and11. A

To obtain the numerical values for the elements of the network of Fig.V10,requation (2 3) is first utilized to. express N? as a power seriesin with the following result:

if 1.2 .13 4113+339@+110,000(,p) 222-10 (ip) (24)l by '20640, and thusare obtained the followlng values for the elements as indicated on Fig.3,

' (#112330 ohms. @l 40.107 mf. (Z1 :4.87 h. 1:7540 ohms.

` I claim:

1. A conductor system for transmitting currents of various frequencies,said system beingsubject to changes of condition 'tending to change thephase relations among the various frequencies, a pilot circuit comprisedin saidsystem, and automatically adj ustablc compensating means`controlled thereby to .keep the -said phase relations constant.

2. A conductor system for transmitting currents of various frequencies,said systemy being' subject 'to changes of condition tending to changethe phase relations among the various frequencies, apilot circuitcomprised 'in said system, and compensating means` controlled thereby tokeep the said phase relations constant.

3. A conductor system for transmitting an altegnating current, saidsystem being` subject to changes of condition tending to change theattenuation 'and tending to produce a different phase displacementatgthe receiving end at diercnt times, means responsive to such changes,and compensating means controlled thereby to keep the attenuationsubstantially constant and to keep the phase displacement substantiallyconstant.

4. Al conductor system Jfor transmitting an alternating currentoffseveral components, .said system being -subject to changes ofvcondition tending to change the phase relations of said components atthe receiving end, a pilot circuit comprised in said system, andcompensating means controlled thereby to keep the said phase relationscon- Lli) CII

stant.

In a multiplex carrier current 'system subject to changes of conditiontending to change the phase relations among the ele- Lineiitary currentsof the system, a pilot cir- 7. A cable comprising a plurality (if confductor pairs, potentiometers interposed in these pairs respectively, oneof thesaid conductor pairsbeing appropriated 4to a pilot circuit, andmeans responsive to a variation of current in ,the pilot circuit toadjust. the said potpntiometers, the steps of these.A

potentiometers being formed to keep the phase displacement constant inthe circuits.

comprising the said pairs of conductois.

A8. A circuit comprising a potentiometer consisting of impedancesteps,each step comprising' a resistance element and a reactance element.so proportioned that at all adjustments the phase relationsamongfcurreiit components ot diierent frequency in the output ofthe'circuit are kept substantially'.` constant fora constant'relationamong these components 'in the input of the circuit. a s

E). A potentiometer consistingof iinpedance steps, each step comprisinga resistance to changes of condition tending to Ichange the currentthrough said circuit, Aand the steps of said potentiometer beingconstructed so that its adjustment .will compensate for phasedisplacements producedby such change of condition.

l0. In combination, a transmission line subject to a change ofresistance due Ito temperature changes along its length, a pilotline'exposed like the transmission line to the same temperatureinfluences, a potentiometer in the transmission line having stepsadapted to change the 'phase relations of currents ot differentfrequency in that line, and means controlled by current in the saidpilot circuit to adjust'said potentiometer to compensatet'or phasedisplacements occasione by the tempei'atiire changes. f

.11. llllie inethod,ot` maintaining the phase relations among aplurality of alternating currents subject to varying conditions at acertain part of their circuit er circuits, which consists in passingthose currents through a potentiometer adapted to 'give Various phasedisplacements at various adjustments and automatically adjusting saidpotentiometer in accordance with the change of the conditions referredto.

12. In combination, a transmission line subject to attenuation changesand phase displacement changes from time to timefa potentiometercomprising resistance ele-4 ments' and reactance elements in its stepsand adapted at its Various adjustments to compensate for such changes,and an associated pilot circuit cont-rolling said potentiometer.

13. rIhe method of maintaining denite phase relations at the receivingend in a multiplex carrier current system, which con-` sists in testingtemperature conditions at points along the transmission line andadjusting for amplitude and phase relations as determined by theseconditions in order to keep the amplitude and phase relationssubstantially unchanged at the receiving end.

14.1,'I`he method of maintaining'definite phase relationsfat thereceiving end in a multiplex carrier current system, which consists intesting temperature conditions at `points iii a pilot circuit along thetransmis- February 1924.

HARRY' NYQUIST.

